1. Field of the Invention
This invention relates to a method of forming a zinc collar on an insulator metal cap and a mold therefor.
2. Description of the Prior Art
Suspension insulators are generally used in the form of insulator string comprising a multiplicity of serially connected insulators interposed between transmission lines and the arms of steel towers for supporting the transmission lines in order to secure insulation to the earth. However, if the surfaces of these suspension insulators are polluted and wetted, leakage current flows over the ceramic surfaces of the suspension insulators, whereby the metal caps undergo electrolytic corrosion to cause thinning thereof. Accordingly, the metal caps come to have reduced strength and they may occasionally be damaged by the load of the transmission lines.
With a view to overcoming the above problems, a suspension insulator, for example, of the structure shown in FIG. 4 has been proposed. This type of conventional suspension insulator has a pin 2 in the cavity of the head 1a of the insulator body 1 and fixed with a cement 3, and also has a metal cap 4 having a socket 4a with which a pin 2 of another insulator unit can be engaged is fixed with a cement 5 over the circumference of the head 1a of the insulator body 1, wherein a zinc collar 6 is integrally formed on the metal cap 4 from the lower external circumferential edge to the bottom for the purpose of preventing such electrolytic corrosion of the metal cap 4.
In forming such zinc collar 6, the following method has conventionally been employed, wherein a metal cap 4 molded through casting of a metallic material such as iron is subjected to pretreatment (degreasing and acid washing) and then to galvanizing, followed by solidification of the thus deposited molten zinc with water cooling. The thus treated metal cap 4 is then dipped upright in a molten zinc 11 as shown in FIG. 5 so that approximately the lower half of the entire cap height may be immersed in the molten zinc 11, and removed therefrom to allow approximately the lower half of the metal cap 4 may be soaked with the molten zinc. Subsequently, as shown in FIG. 6, the metal cap 4 is set on a preheated mold 12 which can be separated into halves. A molten zinc 13 is poured from a sprue 12b of the mold 12, which passes through a gate 12c and flows into a zinc collar molding cavity 12a, followed by solidification of the molten zinc 13 to form a zinc collar 6 on the metal cap 4 from the lower external circumferential edge to the bottom.
Nevertheless, in the above conventional zinc collar forming method, the mold requires a high-accuracy approaching/separating mechanism, since the zinc collar 6 is designed to be formed using a pair of separable die halves, so that the mold assembly comes to have an extremely complicated structure. Moreover, since when the metal cap is released from the mold, the solidified zinc is snatched off at the gate 12c, burrs are formed on the zinc collar surface along the gate 12c, requiring intricate procedures such as deburring and subsequent finish polishing. Further, the molten zinc 13 also stays in the sprue 12b and the gate 12c, extra amount of zinc must be used. For such reasons, production costs inevitably jump up disadvantageously.
In the conventional molding method, the zinc collar molding cavity 12a of the mold 12 has a closed structure, so that the solidification of the molten zinc 13 poured into the cavity 12a proceeds from the external and internal circumferential surfaces of the zinc collar 6 toward the internal portion thereof. Thus, voids (micro-pores) are liable to be formed in the internal portion of the zinc collar 6 and products can be formed in very low yield, disadvantageously.